44 - More Mixer Testing 

Thanks Nick! Well done. The diode thing was very useful, but what really got me was your explanation of why we homebrew -- we want to know how it works! 73 Bill N2CQR

I guess that explains why using a diode tester on a multimeter is 'good enough' in most cases, and my rigs built over the years have worked. Well done for taking the time to do those experiments Nick, and thanks for sharing.

Nice work Nick, The matching as you show makes a big difference on the various spurs, (I don't call reducing the spurs by 6-10db insignificant ! )which can hover well above the noise floor, and they will cause "birdies" if the rx is sensitive enough. I agree that it is the construction that makes all the difference. Andy

👍great work,

Fascinating - thank you Nick! Like you, I expected diode matching to make a big difference. 73 Ian G4COL

Well that was both interesting and surprising! It does, however, prove that good standards of construction are (or should be) top of the list. Nice one matey!

Keep up the good work, very interesting. However schottky diodes have a lot of leakage compared to silicon diodes. I found this out when designing a charge discharge circuit into a mosfet. Maybe it would be better if the leakage was matched.

Good video Nick. You've touched on the what I think is most important point-- symmetry & balance is everything in double balanced mixers. Diodes, the transformers/ baluns + wires + layout -- the lot. Stray coupling (L and C) may also affect balance and I have unwittingly changed these during test runs of various mixer components. The goals of double balanced versus unbalanced or single balanced -- boost inter-port isolation, reduce spurious responses and hopefully improve IP3. I tend to match diodes for port isolation as this is likely the best indicator of mixer balance: e.g. to cancel input signals from outputs at the 3 mixer ports LO is the strongest tone -- so it's easy to measure the LO at the IF ( or RF ) port with the spectrum analyzer. Isolation = LO input available power / LO power at the IF ( or RF ) port. I agree- even in commercial circuits, you may find imbalances that sometimes disappoints for all the $$$$ Different diodes may incur variances in ON resistance and capacitance. Port matching (especially the IF port) and temperature may affect mixer performance measures and that port termination itself is worthy of experimentation. A great mixer (well balanced) with poor port termination may function as a "good mixer". I've found that balanced mixer function tests sometimes vary with frequency -- probably not at HF though. Most mixer data sheets will show port isolation across their intended bandwidth -- we tend to just measure at 1 LO + 1 RF frequency. Strive for balance -- enjoy the results whatever they may be. This the point I get from you. Great thoughts. Thank you. Best! -QRPHB-Todd-VE7BPO- qrp-popcorn.blogspot.com/

Thanks for SA pics, very educational! I saw datasheet for HP matched set of mixer diodes for their SA mixer and they not just match diodes by curve stats but also by capacitance, which is crazy, sadly cant remember datasheet name

Well!, who'd have thought it?!! Thanks for the video and experiment Nick. What a surprise. Given that there is a small advantage, will you be matching in the future, not bother or only in critical applications?

The diodes are just one part of the equation. One that is looked at too often with too much precision for the use case of many amateur radio designs. Perfectly matched diodes are not going to make much of a difference with typically quite asymmetric, hand-wound toroid transformers. Here's an Experiment that's worth the effort: Rewind the transformer on the left side in picture shown at 0:11 , do not solder the midpoint-windings together and do not connect them to ground. Instead take each end of a winding that you would ordinarily solder together, and connect them each, individually to the outer two pins of a low value (10-100 Ohms) potentiometer. The wiper of the pot then should be soldered to ground. You now have a very cheap and easy way to adjust the balance of the balanced mixer. Repeat your modulator test and adjust the pot while viewing the output spectrum. You'll find that you can achieve a significantly higher carrier supression now.

👍Thanks for sharing this Nick. Certainly indicates that your matching device works very well. In terms of matching, I appreciate that you aimed for the best vs. worst case. However, a random batch of x4 is likely to fall into the "reasonably matched" category rather than have one or more "outliers". So, I suspect that x4 random diodes from a batch may give results even closer to the results you got with the matched ones.🤔

Theres one significant detail that's coming to mind here. Those little pads the diodes are on are above the ground. The two tests may have had variations in capacitance to ground, by chance. If you look at the schematics for balanced modulators in older HF transceivers, (FT7B springs to mind) they use a trimmer across the diode bridge for the balance modulator, theres also a preset potentiometer. I hope this gives you fodder for exploration

Cheaper is good if the results are so accuraste and repeatable, I think series connected diodes, anode to anode or cathode to cathode would enable capacitance measurements but from the results here that looks like it's not going to yield much improvement at HF

Your testing was done with a single frequency of a constant amplitude. But vary that audio frequency, vary the carrier frequency and the amplitude, and what do you get? Perhaps more harmonic content, more distortion. The observation about the quality of construction is the key point. I think building a radio is a lot like building a race car. Build each and every single thing as near perfect as you can manage, and when you're done, you'll have a winner. Do a little sloppy work in multiple steps, and you're left with a very sloppy result. In the mixer, everything from lead length to your ground-plane beneath those pads affects the inductance and capacitance of the circuit, inducing resonances, harmonics, and imbalances. If I go to the trouble of building my own transceiver I'm going to match, balance, work and rework, optimize the filters to get the best skirts possible, and do everything I can to build the very best radio I can, whether I can easily notice the difference or not, because that's what it's all about.

I wonder how hard it would be to match BAT54S Diode Array Schottky sets with each other. And wonder if it would make any difference to the uBITX ? At about 19:00 I do see spurs way down but definitely there that are not there on the left. Aha, you did talk about them.

Nice science, Nick! de Phil M1GWZ